Non-metallic inclusions generally reduce the properties of steels, such as reducing transverse mechanical properties, initiating cracks, reducing fatigue life and inducing corrosion. Reducing the number and changing the morphology of inclusions can significantly improve the performance of steels. Therefore, inclusion removal and its morphology control in steel have always been the focus issue. Although the bottom-blown argon, electromagnetic stirring and filtration can remove inclusions to a certain extent, these methods are difficult to effectively remove inclusions smaller than 20 μm in size and cannot effectively control the morphology of the inclusions. Recently, electric current has become a new method for inclusion removal and morphology control. This article briefly reviews the hazards of inclusions and their control methods, and reviews the effects of current on the removal, orientation and morphological evolution of inclusions in metal melts in detail, and introduces three mechanisms of current-controlled inclusion separation: electrophoresis, electrical free energy driving and electromagnetic repulsion. Electrophoresis theory believes that inclusions in the melt are charged, and they migrate parallel to the direction of the current under the action of the electric field force. While the electrical free energy driving and electromagnetic repulsion hold that inclusions migrate perpendicular to the direction of the current. The current waveforms significantly affect the removal efficiency of inclusions. Compared with direct current and alternating current, the pulsed electric current has a stronger ability to remove inclusions, especially pulsed electric current can effectively separate inclusions with a size larger than 5 μm in the molten steel. In addition, the pulse current can not only control the orientation and morphology of the inclusions, but also affect the morphology of the bubbles; the inclusions tend to be refined, spheroidized and arranged parallel to the current under the action of the pulse current. Finally, the research status of current-controlled inclusion separation is summarized, and future research trends are analyzed. At the same time, the application and prospect of pulsed current in anti-clogging of submerged entry nozzle was also analyzed. Due to the low energy consumption, excellent inclusion removal efficiency and easy process equipment, pulsed current separation technique is expected to become a new technology for removing inclusions and suppressing nozzle blockage in the future.
Duplex stainless steels have excellent corrosion resistance, but its insufficient wear resistance limits its application scope. Therefore, alloying of duplex stainless steels to form hard phases, such as carbides, becomes one of the important research directions to enhance their wear resistance keeping their good corrosion resistance. Specifically, carbides are considered as an ideal hard phase to strengthen Fe-Cr-C alloys, where their types, hardness, volume fraction, morphology, size, spacing and interconnecting feature are the important factors affecting the wear resistance of the alloys. It has been shown that, Si as an alloying addition can modify the carbide precipitates in type, phase morphology and distribution in Fe-Cr-C alloys, having an obvious influence on their wear and corrosion resistance as well as their mechanical properties. In this work, the microstructure and properties of two types of ultra-high chromium (40%, mass fraction) and high carbon (1.5%) duplex stainless steels with different concentrations of Si (0.46% and 1.36%) are investigated. The composition, as-cast microstructure and solidification process as well as the changes in the microstructure, phase structure after solution treatment were studied by chemical analysis, OM, SEM, EPMA and XRD. The mechanical properties including hardness, tensile strength, fracture toughness, corrosion resistance and wear resistance have been tested correspondingly. It is revealed that, both the two kinds of duplex stainless steels have a constitution of three phases in as-cast state, i.e. γ phase, σ phase and M23C6. During the solidification process of the steel with 0.46%Si, δ ferrite dendrite forms at the beginning, followed by eutectic (δ+M23C6), peritectic γ, and finally eutectic (γ+M23C6), in which the δ phase transformed into eutectoid (γ2+σ) in the subsequent cooling process. For the duplex stainless steel with 1.36%Si, the increase of δ ferrite amount is observed leading to obviously increased content of σ phase, the morphology of peritectic γ becomes intermittent and irregular shape, and no eutectic (γ+M23C6) forms. After solution treatment, the two kinds of steels are composed of ferrite, austenite and M23C6 type carbide. Note that, the volume fraction and continuity of α ferrite are promoted obviously by increasing Si content from 0.46% to 1.36%. The Si addition slightly improves the hardness, tensile strength and fracture toughness of the duplex stainless steel, while has little effect on the corrosion resistance and slightly reduces the wear resistance.
With the rapid development of high-speed motors, traditional non-oriented silicon steel is difficult to meet its strength requirements. High strength enables resistance to deformation and fatigue fracture induced by centrifugal force. In this work, Nb element is added to traditional non-oriented silicon steel to improve its strength without greatly sacrificing good magnetism. The previous research on Nb-containing high strength non-oriented silicon steel showed that the annealing at high temperature led to good magnetic properties but poor mechanical properties. In order to improve the strength of the steel, the annealing temperature was decreased to make part of the dislocation structure retained in the cold rolled material. The influences of annealing below 900 ℃ on the microstructures, texture, magnetic and mechanical properties of cold rolled Nb-alloyed non-oriented electrical steel were investigated in this work. The increase of annealing temperature promoted recovery at 700~750 ℃ and led to a partial recrystallization with higher fraction at 800~850 ℃; meanwhile, α texture component was enhanced but γ texture suppressed with the increasing temperature. In contrast, the annealing at 900 ℃ resulted in a complete recrystallization, stronger γ but weaker α texture component. Higher annealing temperature produced lower strength and higher ductility as expected, due to dislocations annihilated by recovery and recrystallization, which also led to lower high-frequency iron loss. The value of magnetic induction B50 corresponds well with the intensity of α texture in the annealed steel, and reaches the maximum value at 850 ℃ due to the most intense α texture formed, at which the best combination of mechanical and magnetic properties is also achieved, including the value of magnetic flux B50 (1.572 T), high-frequency iron loss P1.0/400 (33.26 W/kg) and yield strength about 600 MPa, the latter is attributed to the multiple strengthening mechanisms including dislocation, precipitation and grain refinement strengthening.
In the casting components of superalloys with increasing content of the refractory elements the occurrence of sliver defects becomes rather frequent. In comparison to the other grain defects such as stray grains and freckles, the sliver defect was less well understood and its formation mechanism is still unclear. In the present work, the origins of the sliver defects in the single crystal (SC) castings were investigated. In the metallographic detection, sliver grains can be identified to be miss-alignments of isolated, individual primary dendrite on the SC matrix. The sliver defects originated from the tearing of the existing dendrite stems in the mush zone, revealing a clear starting point of sliver defect. The cracks of the dendrites were filled by the interdendritic residual melt, which finally solidified into γ′-stripes. The tearing of some existing dendrite stems can be attributed to the adhesion of shell mold that hinders the shrinkage of the columnar dendrites on the casting surface. The second reason for the dendrite tearing is the insertion of oxide residues which significantly weakens the strength of the dendrite stems. Due to the support of the neighboring columnar dendrites, the tilting of the broken dendrites is limited, so that the grain boundary between a sliver and the matrix structure has normally a low angle. The structure and formation mechanisms of sliver defects were discussed in comparison with other defects such as stray grains, freckles and low angle grain boundaries. The corresponding methods were proposed to avoid sliver defects in production of SC superalloy castings.
As the excellent combination of strength and ductility, the high manganese steel has been used in the manufacturing field of automobile, liquefied natural gas (LNG) ship and oil and gas exploitation. On the other hand, due to the good damping capacity within a certain Mn content range, it has also been used to make components on the machines to reduce vibration and noise. So high manganese steel is considered to be a structural and functional integrated material with great application prospects. Many factors can affect the mechanical properties and damping capacity, such as chemical composition, grain size and heat treatments. Among these, carbon concentration has a complicated influence on them. For example, a high carbon concentration will improve mechanical properties, but in return deteriorate damping capacity. In order to acquire a material with good damping capacity and suitable strength and ductility, ultralow carbon Fe-19Mn-0.0017C (mass fraction, %) alloy was designed. The microstructural evolution and mechanical properties of the alloy during tensile process were investigated by means of OM, EBSD, TEM, XRD and tension test. The results show that Fe-19Mn shows deformation-induced martensite transformation, which changes from γ-austenite→ε-martensite transformation to ε-martensite→α'-martensite transformation as the amount of deformation increases. Analysis of the strain hardening rate (ln(dσtrue/dεtrue)) combined with the fraction of constituent phases reveals that the transformation of ε-martensite→α'-martensite is more effective in improving work hardening rate than that of γ-austenite→ε-martensite. This is, on one hand, because of the lower strength of ε-martensite which is caused by the lack of carbon solution strengthening; and on the other hand, α'-martensite has higher hardness than ε-martensite, which can impede dislocation movement more effectively. In addition, {101?2}<1?011>ε deformation twins are formed to accommodate deformation of ε-martensite except for dislocation slip during tensile process. The combined action of transformation induced plasticity (TRIP) effects of γ-austenite→ε-martensite→α'-martensite transformation, dislocation slip of γ-austenite/ε-martensite/α'-martensite and {101?2}<1?011>ε deformation twinning makes Fe-19Mn with ultralow carbon concentration have an excellent combination of strength and ductility, whose tensile strength and total elongation can reach 722 MPa and 31%, respectively.
Fe-Cr-Al alloy is a promising candidate accident tolerant fuel (ATF) cladding material. It is helpful to developing Fe-Cr-Al alloy with excellent corrosion resistance by investigating the effect of chemical composition on its corrosion behavior under the normal condition of simulated nuclear reactor operation. In this work, the effect of Nb content (0.5%, 1.0%, 2.0%, mass fraction) on the corrosion resistance of Fe22Cr5Al3Mo alloys in 500 ℃ and 10.3 MPa superheated steam was investigated by static autoclave tests. The microstructures of the alloys and oxide films formed on the alloys after different exposure time were observed and analyzed by EBSD, TEM, EDS and SEM. The results showed that the second phase particles (SPPs) of Nb(C, N), Fe2(Nb, Mo) and Cr2(Nb, Mo) were precipitated and the grains were finer in the Nb-containing alloys. The corrosion resistance of the alloys was further improved with the increase of Nb content (in the case of Nb≥1.0%). The oxides of Fe, Cr and Al were formed separately in the oxide layer from outside to inside. Compared with the Nb-free alloy, the interfaces of different oxides were clearer and had a more obvious stratification resulting from the oxides with different composition appeared in the oxide film on the alloy with 1.0%Nb. The thickness of oxide film on the alloy with 1.0%Nb was more uniform than that on the alloy without Nb. At the oxide film and metal matrix interface, the alumina layer on the Nb-free alloy was dispersive, and the alumina particles were detected in both the matrix and chromium oxide scale, which illustrated the internal oxidation of aluminum occurred. While the alumina layer on the alloy with 1.0%Nb was more uniform and continuous. This indicated that the addition of Nb inhibited the internal oxidation of aluminum, and promoted the formation of uniform and continuous alumina layer, therefore reduced the oxidation rate of the alloy.
In recent years, high-entropy alloys have triggered broad research interests due to their unique and intriguing mechanical properties. In general, the increase in strength is accompanied by the reduction in ductility. Therefore, strong and ductile metallic materials have always been pursued by metallurgist. Heterogeneous structure has been reported to be very useful for overcoming the strength-ductility trade-off in metallic materials. In this work, typical partially recrystallized structure has been obtained in CoCrFeNiMo0.2 high-entropy alloy by cryogenic rolling and annealing. The effect of partially recrystallized structure on the mechanical properties has been studied. After 35% cold rolling (RTR35%) and 35% cryogenic rolling (CTR35%) and annealed at 800 ℃ for 30 min, CoCrFeNiMo0.2 high-entropy alloys developed partially recrystallization microstructures featured by coarse deformed grains and fine recrystallized grains. The yield strength of the CTR35% sample is 539.3 MPa and its elongation is 46.8%, which is similar in strength but 30% higher in elongation when compared with the RTR35% sample. This can be understood from the fact that samples rolled at cryogenic temperature showed a higher volume fraction of fine recrystallized grains, resulting in better strain hardening capability.
The poor plastic deformation ability of magnesium alloy, resulted from its close-packed hexagonal structure and only two independent basal <a> slip systems at room temperature that cannot meet the von Mises criterion, has extremely restricted its application. As the α-Mg dendrites grow along with the heat flow in directional solidification, the uniform columnar crystal structures obtained in Mg can effectively improve its mechanical properties. And the mechanical properties of the anisotropic magnesium alloys were heavily affected by the orientation controlled by the directional solidification parameters. In this work, the effects of Gd content (3.0%, 4.5%, 6.0%, mass fraction) on the microstructure and mechanical properties of directionally solidified Mg-xGd-0.5Y alloy were investigated. The tensile deformation behavior at room temperature was analyzed by EBSD technique. The results showed that the Mg-xGd-0.5Y alloys have a longitudinal grain boundary parallel to the heat flow direction and a preferential growth along the normal direction of the (112?0) plane at a withdrawn rate of 3 mm/min. The cross section of the columnar crystal was triangle or crisscross petal in shape, and the secondary branch gradually changed from three branches of 3.0%Gd to four branches of 6.0%Gd. The Mg-6.0Gd-0.5Y alloy with more columnar crystal growing along with <224?3> direction had higher tensile strength (107 MPa) and post-break elongation (32.56%) at room temperature, and its deformation mechanism was basal <a> slipping and {101?2} extension twinning. When the crystal growth directions dispersed (concentrated on the <1?21?0> and <224?3>) in the Mg-3.0Gd-0.5Y alloy, it had low post-break elongation (14.88%) because of poor synergistic deformation ability, which have {101?2} extension twins and {101?1} contraction twins to accommodate strain.
With the development of science and technology, more and more products with excellent quanlity and abundant functionalities have been exploited and provided. Inspired by the concept of "brick wall" structure or layer structure with alternated distribution of hard and soft phases discovered in nature creatures such as mother pearl shellfish, a entirely novel steel composite which not only can minimize the shortcomings of the original materials at the maximum extent, but also possess excellent mechanical performance as well as new physical properties, has been developed. Taking ultra-high strength maraging steel and 316L austenitic stainless steel as the original materials, the influence of deformation reduction under high vacuum on interfacial bonding strength and interface characteristics of heterogeneous multi-layered metal composites was studied, and the fabrication feasibility of heterogeneous multi-layered metal composites was explored. The results showed that in the vacuum hot-pressing process, the interfaces under different deformations were clear and straight. Slight mutual diffusion phenomenon occurred in the hot-pressing process. Due to the difference of rheological properties of the original materials at high temperature, dynamic recovery and dynamic recrystallization occurred in the 316L layer, while deformed microstructure was dominant in the maraging steel layer. Combined with rolling process and heat treatment, bulk metal composites with 9 layers and 11 layers were prepared, respectively. The results of the three-point bending experiment showed that the crack occurred firstly at the outermost side of the multi-layer composites which withstood the tensile stress. Due to the passivation, delamination and bridging of heterogeneous interface in the multi-layer metal composites, the propagation path of crack was greatly extended and more energy was consumed, which showed excellent ability to block the crack propagation.
The pipe joints based on shape memory alloy (SMA) are widely used in various fields of fluid transport by virtue of their simple structure, easy assembly and high reliability. However, due to the complexity of the NiTiNb constitutive model, the plastic deformation and its effects have yet not been considered in the report of pipe joint connection system. In view of this background, this work constructs an SMA joint-steel pipe system (J-P system) and performs the finite element numerical simulation of the assembling process based on an SMA phenomenological constitutive model, where in the plastic-phase transformation coupling effect is considered. By altering the diameter expansion, wall thickness, service temperature and critical phase transformation, the change features of the von Mises stress, contact pressure and pull-out force of the J-P system are investigated. The results show that due to the coupling effect of phase transformation and plastic deformation, the evolution of Mises stress, equivalent transformation strain and equivalent plastic deformation in SMA joint show obvious regularity during assembly: in the loading stage, the phase transformation strain and plastic deformation increase with the increase of predeformation. At each subsequent loading step, the plastic strain remains unchanged. At the unloading stage, von Mises stress decreases and phase transformation strain remains unchanged. With temperature increase, the phase transformation strain decreases significantly and von Mises stress increases. At subsequent loading steps, von Mises stress and phase transformation strain remains unchanged. Within a certain size, the pull-out force decreases with the increase of diameter expansion; Among the 9 schemes with different wall thickness ratios, the pull-out force changes non-linearly with the wall thickness, and there is an optimal connection performance scheme. Within the range of room temperature (0~40 ℃), the service temperature has little impact on the performance of the J-P system; With the increase of the critical phase transformation, the stress concentration layer within the SMA joint moves from the inside to the outside, and the pull-out force increases gradually within the range of the critical phase transformation from 0.07 to 0.14. The results also show that the stress concentration at the end of contact region can significantly increase the pull-out force of the J-P system.
Selective laser melting (SLM) is a very promising additive manufacturing (AM) technology for fabrication of thin-walled parts due to its high forming accuracy with complex shape. The higher temperature gradient in rapid heating and cooling process is prone to produce larger thermal stress, which will induce warpage deformation of SLMed parts. However, most of the current SLM stress studies focus on the residual stress, and only a few reports on the transient stress in the thermal cycle during SLM. In this work, a thermal-mechanical coupled transient dynamic finite element model was established to study the effects of laser scan rate and layer thickness on stress evolution during SLM processing. The results show that under the action of thermal cycle, the internal stress evolution in SLM of titanium alloy thin-walled parts presents a thermal stress cycle. Under the relief annealing of the thermal stress cycle, the peak thermal stress increases first and then decreases in the heating stage, and stabilizes and approaches the value of residual stress in the cooling stage. The residual stress of SLMed thin-walled parts is less than the transient peak stress during heating. After several thermal cycles with stress relief annealing effect, the peak thermal stress of SLM thin-walled parts can be reduced by more than 30%.
